Sulphur-modified polychloroprene rubber has many valuable properties in terms of application and has therefore found a wide field of application. Thus, its ability to be masticated which is greater than for the so-called mercaptan types along with its ability to be vulcanised without the addition of thiourea accelerators are of particularly advantageous value. Moreover, articles which are to be subjected to great dynamic stresses, such as V-belts, are preferably produced from this special type.
The copolymerisation of chloroprene with sulphur in aqueous-alkaline emulsion, which copolymerisation is triggered off by radicals, has been known for a long time and is described, for example, in "Ullmans Encyclopadie der Technischen Chemie", Vol. 9, P. 366 et seq, Verlag Urban & Schwarzenberg, Munich-Berlin 1957, and in "Encydopedia of Polymer Science and Technology", Vol. 3, P. 705 to 730, John Wiley, N.Y. 1965.
The polymer obtained by copolymerisation has a high molecular weight and can hardly be processed with conventional equipment on account of its very poor plasticity.
The molecular weight can be reduced and the desired plasticity can thereby be achieved by suitable compounds which are present during polymerisation or which are added to the latex subsequent to polymerisation. These technics are described, for example, in U.S. Pat. Nos. 35 07 825, 22 64 173, 33 78 538, 33 97 173, in German Offenlegungsschriften Nos. 21 34 158, 20 18 736, 22 13 116, 20 03 147, 19 11 439, 18 07 298, 22 41 394, in French Patent Nos. 14 57 004 and 14 80 110, and in the British Patent No. 9 59 122.
In the two-stage processes, a latex is firstly produced, whose polymer is adjusted to the desired viscosity in the second stage by addition of a peptisation agent, such as tetraethylthiuram disulphide (TETD).
This process for the production of sulphur-modified polychloroprene types with stable viscosity and which are stable in storage is described, for example, in DE-OS 22 18 152, DE-OS 22 13 116, DE-OS 20 03 147 and GB-12 19 782.
DE-OS 20 18 736 also describes a process in which TETD in conjunction with dibutylamine is added to the latex on completion of polymerisation. The amine serves to accelerate peptisation.
DE-OS 20 03 147 describes a process in which the presence of water-soluble tertiary amines favourably influences polymerisation. In the above processes a highly polymeric polychloroprene is produced which is analytically difficult to characterise and thus makes the subsequent peptisation process erratic and difficult to reproduce.
There have been many attempts to produce sulphur-modified polychloroprene having a favourable range of viscosity directly in a one-stage process.
In this process, the desired plasticity is adjusted using so-called regulators, such as xanthogen disulphides, iodoform or C.sub.8 -C.sub.20 alkylmercapatans. These processes are described, for example, in U.S. Pat. Nos. 33 78 538, 33 97 173, FR-14 57 004 and in DE-OS 19 11 439.
Good results have hitherto been achieved, according to DE-PS 26 45 920, by adding a secondary and/or tertiary aliphatic amine having unbranched and/or cyclic alkyl chains with from 2 to 18 carbon atoms in a quantity of from 0.01 to 1 parts by weight per 100 parts by weight of monomers and optionally a tertiary aliphatic water-soluble amine having hydroxy groups in a quantity of up to 0.45 parts by weight per 100 parts by weight of monomers to the monomers to be polymerised, adding tetraethylthiuram disulphide either in a quantity of from 0.05 to 1.0 parts by weight per 100 parts by weight of monomers to the polymerisation mixture after a monomer reaction of at least 5% and at most 25%, or adding tetraethylthiuram disulphide in a partial quantity of from 0.05 parts by weight to 0.40 parts by weight to the mixture before commencement of polymerisation and adding specific emulsifiers or emulsifier mixtures.
This known process requires a large number of additional chemicals which have to be added in quite specific quantities and at definite times during polymerisation, although they are not advantageous for the finished rubber in which they remain and are possibly even disadvantageous depending on the field of use.
Thus, an object of the present invention was to provide a process for the production of sulphur-modified polychloroprene which eliminates a subsequent peptisation stage and the disadvantages of the known process.